This invention relates to circuits and methods that enable multiphase or polyphase switching regulators to evenly divide load current between stages (phases) without the use of a current sense resistance or relying on the analysis of switches such as MOSFET devices. More specifically, this invention relates to systems and methods that monitor the time varying voltage change of the input capacitor in multiphase switching regulators and utilize this monitored value to control load sharing.
DC to DC regulator circuits (power converters) are invaluable in situations where the need is present to regulate an input voltage, such as voltages from household power outlets, for a particular product or application. Power techniques present in such DC to DC regulators can solve the problems surrounding fluctuating and poorly-specified input signals. By producing a desired output voltage, either higher or lower than the regulator's input voltage, DC to DC regulators can control and vary an output voltage with respect to time.
When particularly high power levels need to be delivered to an output, multiple power converters can be placed in parallel configurations by utilizing a common input and output node. Parallel configurations can essentially increase the amount of current that is delivered to the common output node, thus increasing the regulator's potential output power. The switching characteristics of multiple parallel power converters can also be shifted in time (phase) with respect to each other. If shifted properly, the power converters can ultimately divide and evenly share a common input current while maintaining a desired regulated output current. Advantages obtained by such multiple phase regulators include reduced input and output ripple currents. As a result, the size of the input and output capacitors, in addition to the size of the inductors utilized by the converter stages, are reduced.
The voltage-mode multiphase switching regulator method is one technique for controlling load current. This method utilizes the output voltage to control the amount of time, or duty cycle, that a converter stage's main switch is ON. By turning a converter's main switch ON, current is allowed to flow from the regulator's common input node, through the individual converter's inductor, to the regulator's common output node. By controlling the amount of time that a converter's main switch is ON, the amount of current delivered to the common output of the regulator is controlled. Examples of multiphase switching regulators that employ the voltage-mode control technique include Semtech's SC1144 of Camarillo, Calif. and National Semiconductor's LM2639 of Austin, Tex.
One disadvantage of a voltage-mode multiphase switching regulator exists in the difficulty to control currents in each individual power stage. Slight differences in switch ON-times between phases can result in significant inequality in load sharing between stages. Additionally, as a result of the unequal parasitic resistances that exist between each individual stage's MOSFET switches and inductors, the average inductor currents in such a regulator are unequal. In order to fix such inequality errors, significantly larger MOSFET switches and inductors must be used to accommodate the unequal load sharing. However, this solution to the power distribution problem increases production costs and reduces power efficiency.
One alternate approach employs a sense resistor at the input of each individual converter stage. The current across an individual sense resistor may be monitored and used to control the duty cycle of the respective converter stage. In changing the duty cycle of the individual converter stage, the distribution of load current throughout the switching regulator is regulated. One example of a voltage-mode multiphase switching regulator that employs input current sense resistors to balance inductor currents is Linear Technology's LTC1702 of Milpitas, Calif.
Regulators that utilize sense resistors for balancing inductor current within the individual converter stages, however, still have disadvantages. For example, current sense resistors cause additional dissipative losses in regulators which, in turn, reduce operating efficiency; the ratio of the power supplied to power provided to a regulator (where a ratio of unity, 1/1, is ideal). Yet, in order to prevent dissipative losses from becoming excessive, sense resistors must have a very low resistive value with typical values residing in the 0.001Ω to 0.1Ω range. These sense resistors must have good matching characteristics. Resistors with low resistance values and good matching characteristics, however, are costly and difficult to manufacture. These regulators also must employ a capacitor, in addition to the sense resistor, for each converter circuit, thereby increasing production costs and design complexity. Finally, sense resistors are physically large and require valuable circuit board area. For at least portable product designs, circuit board area is limited, thus limiting the amount of portable applications in which switching regulators with sense resistors may be beneficial.
Another known technique helps match average inductor current between stages while eliminating the use of sense resistors. The technique uses the ON-resistance of each converter's power MOSFET switches to determine the amount of current flowing through the individual stages. Examples of voltage-mode multiphase switching regulators that use on-board MOSFET switch resistance to control and balance current sharing throughout a regulator are Harris' HIP6301, HIP6302, and HIP6303. The matching characteristics, however, of power MOSFET switches are poor, typically having a loose specification in the +/−30% range. Such matching characteristics are significantly worse than those found in sense resistors. Moreover, there is no guarantee that matching can be achieved between the parts of each individual MOSFET switch. Such disadvantages make MOSFET sense regulators problematic and deficient.